Super absorbent polymers (SAP) are compounds capable of swelling to many times their original weight by absorbing water. SAPs are used, among other things, as an absorbing compound for baby diapers, in protecting power and communication cables, in agriculture for use in increasing the capability of soil to retain moisture and nutrients, and in the hygienic packaging of food products with absorbent pads. SAPs swell upon exposure to water, and in many instances, are capable of absorbing up to 500 times their weight in water. The SAPs are typically used in powder form or in a composite form in which SAP particles are blended with fine fibers and then entrapped within a fibrous mat.
In the past, attempts have been made to use the SAPs to form coatings on synthetic fiber used in the manufacturing of cable, such as fiber optic cable. The coated synthetic fiber was used in place of preexisting synthetic fiber components for cable. The intention was that the coating would swell upon exposure to water and fill gaps in the cable rendering the cable watertight to protect the fiber optic component. The present inventors attempted to provide SAPs to the cable in the form of a fully fused solid dispersion of SAP in a polyvinyl chloride (PVC) plastisol coated fiber. The PVC plastisol included PVC resin in suspension in a plasticizer. However, the fully used dispersion in fiber optic cables did not perform satisfactorily, because the SAPs do not swell at a sufficient rate when subjected to pressurized water. The fused, solid PVC plastisol matrix functions to restrain the ability of the SAP to swell.
To form these dispersions, SAP powder was blended with liquid PVC plastisol formulation. The plastisol formulation was converted from its liquid state to a homogeneous solid plastisol through fusion of the plasticizer and PVC at elevated fusion temperatures of about 121 to about 170.degree. C. or more to fully cure the dispersion. Synthetic fibers were coated with the fused dispersion and then used in the manufacture of fiber optic cable as a filler, binder, strength member and similar uses.
A further application of SAP in a coating formulation includes an emulsion of SAP in an oil-in-water emulsion as described in U.S. Pat. No. 5,342,686. This coating formulation may be used for light coatings on KEVLAR substrates, but is not suitable for heavier SAP coatings, such as those required to block penetration of water into coaxial cable. Further, the emulsion uses solvents which may be detrimental in many coating applications and/or harmful to the enviroment.
While the SAP/PVC dispersions for use as swellable coatings were essentially comparable with other prior art fillers, a need in the art remains for an improved SAP coating composition which is capable of using more of the swelling capabilities of the SAPs and expanding the potential uses for SAP coatings. There is also a need in the art for a method of forming an SAP coating composition which provides maximum water absorption properties. In addition, it would be desirable to achieve a coating composition useful on synthetic fibers, synthetic woven and unwoven fabrics and other substrates which swell at a rate sufficient to protect the substrate from penetration of water.